1. Field of the Invention
The present invention relates to a microcapsule for heat-storing material which is used for cooling or heating a substance or for maintaining a substance at a constant temperature. More particularly, the present invention relates to a microcapsule for heat-storing material which has a good fluidity under any temperature condition and a high stability over an extensive period of time, and can retain latent heat at a high density. The microcapsule for heat-storing material of the present. invention can be utilized as a heating and cooling medium for air conditioning, or it can be utilized as a portable high-temperature insulation material or low-temperature insulation material by holding in any of various packaging materials and containers.
2. Related Art
A heat-storing material most generally used every day is water. Water is used for high-temperature or low-temperature insulation usually in the form of hot water or ice and can be said to be the most inexpensive heat-storing material in daily life.
In general, as compared with a method utilizing only sensible heat which does not accompany the phase transitions of a substance, a method of heat storage by utilization of latent heat accompanying the phase transitions has the following advantage. In this method, since a large quantity of thermal energy can be stored at a high density in a narrow temperature range including the melting point, the volume of a heat-storing material can be reduced, and moreover the heat loss can be kept small because no large temperature difference develops, considering the large quantity of heat stored.
As a heat-storing material in which there is utilized latent heat accompanying phase transitions, in particular, phase transitions between liquid and solid, any heat-storing material can be used so long as it has a melting point or freezing point. There is preferably used a heat-storing material which is physicochemically stable and absorbs heat of fusion of 20 kcal/kg or more in practical application. The following materials are generally known as typical heat-storing materials.
(1) Inorganic compounds containing a large amount of water of crystallization, for example, calcium chloride hexahydrate, sodium sulfate decahydrate, sodium hydrogenphosphate dodecahydrate, sodium thiosulfate pentahydrate, and nickel nitrate hexahydrate. PA1 (2) Organic compounds, for example, aliphatic hydrocarbons such as tetradecane, pentadecane, cyclohexane, etc.; aromatic hydrocarbons such as benzene, naphthalene, etc.; fatty acids such as lauric acid, stearic acid, etc.; alcohols such as lauryl alcohol, stearyl alcohol; and ester compounds such as methyl stearate, methyl cinnamate, etc.
For increasing the heat exchange efficiency of these various heat-storing materials, there have been proposed means for encapsulating the heat-storing materials in microcapsules [for example, Jap. Pat. Appln. Kokai (Laid-Open) Nos. SHO 62 (1987)-1452, SHO 62 (1987)-45680, SHO 62 (1987)-149334, SHO 62 (1987)-225241, SHO 63 (1988)-115718, SHO 63 (1988)-217196, and HEI 2 (1990)-258052].
All of the methods for encapsulation in microcapsules disclosed in the above references are capsulation methods in which water or any of the inorganic compounds belonging to the above group (1) is encapsulated in microcapsules. Also in the case of the organic compounds belonging to the group (2), i.e., the organic compounds which undergo phase transitions, namely, which have a melting point, the employment of a conventional capsulation method permits production of an oil-in-water type microcapsule dispersion which is seemingly rich in capsule solidness and fluidity.
When there was produced an oil-in-water type dispersion of microcapsules encapsulating the aforesaid organic compound capable of undergoing phase transitions and a heat-storing operation was carried out, the following problem was found to be caused. The dispersion of microcapsules encapsulating the compound capable of undergoing phase transitions which is usable in the present invention, repeats heat absorption or heat dissipation on heating or cooling, respectively, and can be used for various purposes. It was found that in this case, there is caused a phenomenon that the melting point and freezing point of the compound capable of undergoing phase transitions which has been encapsulated in the microcapsules are different from each other, namely, a remarkable supercooling phenomenon.
It is known that the supercooling phenomenon is usually caused in greater or lesser degree when a compound capable of undergoing phase transitions is cooled to fall into a solid state. In the case of a microcapsule encapsulating the compound capable of undergoing phase transitions, like the microcapsule of the present invention, the supercooling phenomenon is markedly accelerated, so that high energy has been necessary for the phase transitions.
In general, as a method for preventing the supercooling phenomenon, there are known a method using a nucleating agent such as metal powder or clay powder, and a mechanical method in which agitation, slight vibration, application of an electric shock, or the like is carried out. When either of these methods is applied to the microcapsule of the present invention, the former method is disadvantageous in that the encapsulation of the powder in the microcapsule results in the deterioration of wall of the microcapsule and the restriction of particle size of the microcapsule by the particle size of the powder. The latter method, i.e., the mechanical method is hardly effective probably because the compound capable of undergoing phase transitions is completely segregated from the outside of the microcapsule.